The two primary types of random access memories used in data processing systems are the dynamic RAM and the static RAM. The dynamic RAM is often the memory of choice because each memory cell includes fewer circuit elements and a larger density memory can thus be obtained. Also, dynamic memories typically have shorter access times. A disadvantage of such memory devices is that the charge stored on each memory cell must be replenished periodically such as every four milliseconds. However, even with the addition of refresh circuitry, the dynamic RAMs have greater density.
With the introduction of portable computers which operate on battery supplies, the power requirements of memories become a greater factor in system design. Static memories draw less current from the battery and may thus be chosen despite their lesser density and greater access times.
Recently, a new dynamic RAM referred to as a pseudo-static RAM has been developed. The pseudo-static RAM may be placed in a self refresh "sleep" mode in which the dynamic memory is refreshed by refresh signals generated by logic circuitry on the memory chip itself with current requirements of only about 100 microamps. The memory is intended to be accessed while it is in a standby mode which requires up to one milliamp of current. Once accessed, the memory uses 40 to 50 milliamps. In the standby mode, the memory performs as a dynamic RAM and must therefore be periodically refreshed. If the memory chip is accessed while it is in the self refresh mode it must be given sufficient time to change to the standby mode before information can be written into or read from the memory chip.